Telecommunication system and method for wavelength-division multiplexing transmissions with a controlled separation of the outgoing channels and capable of determining the optical signal/noise ratio

ABSTRACT

A multi-wavelength optical telecommunication method includes the steps of generating at least one optical transmission signal in a predetermined wavelength band, transmitting the optical transmission signal through an optical fibre to a receiving station, receiving the optical transmission signal through a passband filter and filtering the signal to let the transmission signal alone pass. The wavelength band is scanned in order to identify in the band a recognizable portion of the optical spectrum being received, thus determining, based on the operating conditions corresponding to the recognizable spectrum portion, a search range of the transmission signal within which the transmission signal is searched out and recognized based on its spectral profile.

This application is a continuation of application Ser. No. 08/561,085,filed Nov. 20, 1995, now U.S. Pat. No. 5,712,716.

DESCRIPTION

The present invention relates to an optical telecommunication system andmethod, particularly adapted for a wavelength-division multiplexing (orWDM) transmission in which the different channels at reception arerecognized and separated, based on the spectral profile of the opticalsignal, and in which measurement of the optical signal/noise ratio ofeach channel is obtained.

For a WDM transmission several channels, or several transmission signalsindependent of one another, are required to be sent over the same lineconsisting of an optical fibre by multiplexing in the domain of opticalfrequencies. The transmitted channels can be either digital or analogand are distinguished from each other because each of them is associatedwith a specific wavelength, separated from that of the other channels.

In order to enable transmission of a great number of channels using theso-called third transmission window of the silica-based fibres and theuseful band in the optical amplifiers, the wavelength separation betweenthe channels themselves is conveniently in the order of nanometers.

For a correct reception of such transmission signals, it is thereforenecessary to carry out a separation between said channels, so as toconvey them to the respective users.

For the purpose, low-band optical filters can be utilized through whichonly the selected channel can pass, so as to ensure the absence ofundesired signals, that would constitute a noise, being overlapped withthe selected channel.

For use of such filters, however, both a high stability in thewavelength of the transmitted signal and a high inherent stability ofthe passband of the filter themselves is required.

The above problem is described for example in Patent Application GB2260046 suggesting to overlap a pilot signal with data to betransmitted, by the detection of said pilot signal the receiver beingable to adjust the filter passband.

Known optical filters, in addition, suffer from shift problems, based onwhich a selected wavelength for the passband keeps constant only over alimited period of time after setting. Said filters, in particular in thecase they are provided with piezoelectric actuators or the like, arealso subjected to hysteries phenomena. Based on said phenomena, theselected value of a passband wavelength not only depends on the value ofthe relevant command quantity (a voltage for example), but also on thetemporal law by means of which such a quantity is applied to the filtersthemselves.

In addition, in an optical telecommunication system the opticalsignal/noise ratio should be checked, at the exit of an amplificationstage for example, while at the same time the same filtering conditionsas applied to the receiver are being applied, so that the systemfunctionality can be checked.

Such a check however, generally can be carried out only with the use ofexpensive laboratory equipments.

Patent Application GB 2272590 discloses a method of measuring thesignal/noise ratio comprising the steps of selectively filtering atransmitted signal with a notch (a narrow-band-pass filter) andmeasuring the signal power within said band for the filtered andunfiltered signal, in order to ascertain the signal/noise ratio in anamplifier.

Upon experiments carried out by the Applicant, however, the use ofoptical filters to this purpose proved to be subjected to the aboveproblems concerning temporal stability and hysteresis and did not giveappropriate results.

In one aspect, the present invention relates to a method and anapparatus for receiving transmission signals in a WDM system, in whichthe passband of an optical filter is continuously checked and adjustedby recognizing a sure reference value in the scanned wavelength bandand, based on this recognition, the fine search range of the desiredchannel is detected, which range is recognized based on its spectralprofile in the absence of check signals overlapped therewith.

In another aspect, the present invention consists in detecting thespectral profile of an optical signal passing through a tunable passbandoptical filter and recognizing the spectral profile thereof, therebydetermining, based on said profile, the optical signal/noise ratio ofthe signal itself.

In particular, in one aspect the present invention relates to an opticaltelecommunication method comprising the steps of:

generating at least one optical transmission signal, at a predeterminedwavelength included in a predetermined wavelength band;

transmitting said optical transmission signal through an optical fibreto a receiving station;

feeding an optical signal comprising said optical transmission signal toa respective one of the receiving units in said station, through apassband filter;

receiving in said receiving unit the optical transmission signal passingthrough said filter;

said optical signal having a spectrum including at least onerecognizable portion at a known wavelength in said predeterminedwavelength band,

characterized in that said filter is a filter tunable to severalwavelengths in a search band including said spectrum, by command meansoperable under several operating conditions, and in that said filteringstep comprises:

scanning said predetermined wavelength band by varying said operatingconditions;

identifying said recognizable spectrum portion;

determining, based on the operating conditions corresponding to saidrecognizable spectrum portion, a search range for said transmissionsignal;

scanning said search range by varying said operating conditions;

recognizing said optical transmission signal in said search range andidentifying the relevant operating conditions; and

maintaining the operating conditions at said optical transmissionsignal.

More particularly, said steps of scanning said predetermined wavelengthband and identifying said recognizabile spectrum portion comprise:

actuating said command means at least at two operating conditionscorresponding to through-wavelengths of said filter included within saidsearch band;

detecting the optical through-power values at each of said operatingconditions;

identifying between said optical power values a value corresponding tosaid recognizable portion of said spectrum and the operating conditionsthereof.

In particular, said step of determining a search range comprises thestep of determining, starting from the operating conditions of saidrecognizable portion, new operating conditions corresponding to a rangeof said spectrum in which said optical transmission signal can beindividually present and applying said conditions to said actuators.

In a preferred embodiment, the optical telecommunication methodaccording to the invention comprises amplifying said signal at leastonce by at least one active-fibre optical amplifier having aspontaneous-emission spectrum in said band including at least one peakof known wavelength constituting said recognizable spectrum portion.

Preferentially, said filter is a tunable optical filter and said commandmeans is embodied by piezoelectric actuators.

In a preferred embodiment, said filtering step comprises:

applying to said actuators two or more piloting voltages includedbetween the extreme values of predetermined voltages;

detecting the optical power values of the signal passing through thefilter at said voltages;

recognizing said recognizable spectrum portion and the piloting voltagecorresponding thereto;

modifying said predetermined extreme voltage values depending on thevalue of said piloting voltage corresponding to said recognizableportion;

repeating the cycle a predetermined number of times;

determining a search voltage range for a signal;

recognizing said signal in said range; and

maintaining said filter at said signal.

Preferably, said recognizing step comprises:

applying to said actuators piloting voltages included in said searchvoltage range and detecting the optical through-powers correspondingthereto; and

recognizing as a signal each maximum value of the optical through-power.

In addition and in particular, said maintaining step comprises applyingto said actuators the piloting voltage corresponding to said recognizedmaximum of optical power and periodically varying said voltage accordingto predetermined increments, by adopting the piloting voltage valuecorresponding to the detected maximum of optical through-power.

Preferentially, said filtering step comprises varying the pilotingvoltage between said extreme values by means of a predetermined temporallaw. Preferably and in particular, said piloting voltage is variedaccording to increments fixed in time and, more preferably, saidpiloting voltage is varied according to a mean temporal gradientpredetermined in each step.

In particular, said step of determining a search range comprisesdetecting a spontaneous-emission spectrum, identifying the operatingconditions corresponding to the extremes of said spontaneous-emissionspectrum and calculating the operating conditions corresponding to oneportion of said spectrum in which said transmission signal can beindividually localized.

In one aspect of the method of the present invention, said step ofrecognizing said optical transmission signal comprises:

detecting the optical power passing through the filter in a group of atleast three consecutive operating conditions;

separating an optical through-power value detected at an intermediateoperating condition between said consecutive operating conditions, fromthe optical through-power values detected at least at two externaloperating conditions, between which said intermediate condition isincluded;

calculating an optical interpolation power value at said intermediateoperating condition;

comparing said detected optical through-power value with said opticalinterpolation power value; and

recognizing as the operating conditions corresponding to the opticaltransmission signal, the intermediate operating conditions in which saiddetected optical through-power value and said optical interpolationpower value are in a predetermined relation with respect to each other.

In a particular embodiment, said predetermined relation comprises ahigher ratio between said optical through-power value and said opticalinterpolation power value than a predetermined threshold value.

Alternatively, or in addition, said predetermined relation comprises aratio between the integral of an interpolation curve of said opticalthrough-power values detected at said consecutive operating conditionsand the integral of an interpolation curve of said detected opticalthrough-power values, except for the value or values corresponding tosaid intermediate operating condition or conditions in said group, saidratio being higher than a predetermined threshold value.

Preferably, said active-fibre amplifier comprises an erbium-doped fibreand, in particular, said recognizable spectrum portion consists of aspontaneous-emission peak of erbium, at a wavelength included between1530 and 1540 nm.

In a second aspect, the present invention relates to a method ofmeasuring the signal/noise ratio for a predetermined transmission signalover an optical telecommunication line, characterized in that itcomprises drawing a fraction of the transmitted optical signal,filtering said optical signal through a tunable filter while detectingthe optical through-power in a predetermined wavelength band includingsaid transmission signal and comparing the optical power values detectedat the wavelengths of said signal with the optical power valuesinterpolated at the same wavelengths.

In particular, said method comprises:

detecting the optical power passing through the filter in a group of atleast three consecutive operating conditions;

separating an optical through-power value detected at an intermediateoperating condition between said consecutive operating conditions fromoptical through-power values detected at least at two external operatingconditions, between which said intermediate condition is included;

calculating an optical interpolation power value at said intermediateoperating condition;

comparing said detected optical through-power value with said opticalinterpolation power value;

recognizing as the operating conditions corresponding to the opticaltransmission signal, the intermediate operating conditions in which saiddetected optical through-power value and said optical interpolationpower value are in a predetermined relation with respect to each other;and

defining as the signal/noise ratio of said transmission signal, a ratiobetween values resulting from said detected optical through-power valueand said optical interpolation power value.

Preferentially, said predetermined relation comprises a ratio betweensaid detected optical through-power value and said optical interpolationpower value higher than a predetermined threshold value.

Alternatively, said signal/noise ratio consists of the ratio between theintegral of an interpolation curve of said optical through-power valuesdetected at said consecutive operating conditions and the integral of aninterpolation curve of said detected optical through-power values, withthe exception of the value or values corresponding to said intermediateoperating condition or conditions in said group.

In a third aspect, the present invention relates to an opticaltelecommunication system comprising:

an optical-signal-transmitting station comprising means for generatingtransmission signals at at least two wavelengths included in apredetermined bandwidth and means for conveying said signals to a singleoptical fibre line,

a receiving station for said optical signals, and

an optical fibre line connecting said transmitting and receivingstations,

characterized in that said optical-signal-receiving station comprisesmeans for separating said transmission signals from said single opticalfibre line, comprising:

a signal splitter designed to split the incoming optical signal ontoseveral optical outlets;

at least one tunable optical filter connected in series with at leastone of said optical outlets, adapted to produce an optical output signalin a wavelength band of predetermined width and comprising respectivecommandable actuator means;

means for receiving at least one portion of said optical output signalfrom said filter; and

means for commanding said actuator means of said filter, in connectionwith said receiving means.

Preferably, the system according to the present invention comprises atleast one active-fibre optical amplifier interposed along said opticalfibre line.

Preferentially, said amplifier is an erbium-doped active fibreamplifier.

In one embodiment of the system according to the present invention saidtunable filter is a filter of the Fabry-Perot type.

Preferably and in particular said receiving means is designed to receiveat least one portion of said outgoing optical signal from said filterand comprises a fused-fibre splitter, connected in series at the filteroutput, having an outlet connected with an optical check receiver.

More preferably, said splitter draws less than 5% of optical power to besent to said optical check receiver.

In particular, said optical receiver comprises a photodiode for theelectronic detection of the optical signal.

In particular, said Fabry-Perot filter has a free spectral range FSRgreater than or equal to the spontaneous-emission band of saiderbium-doped active fibre.

In a fourth aspect, the present invention relates to a device for amulti-wavelength optical reception, characterized in that it comprises:

a signal splitter adapted to split an incoming optical signal ontoseveral optical outlets;

at least one tunable optical filter connected in series to at least oneof said optical outlets, adapted to produce an optical output signal ina wavelength band of predetermined width, comprising respectivecommandable actuator means;

means for receiving at least one portion of said optical output signalfrom said filter; and

means for commanding said actuator means for said filter, in connectionwith said receiving means.

In particular, the command means for said filter actuators comprises amicroprocessor unit adapted to generate a command action on theactuators in response to the filter output signal.

In particular, said actuators for said filter are piezoelectricactuators.

In a preferred embodiment, said tunable filter is a filter of theFabry-Perot type.

Preferentially, said means for receiving at least one portion of saidoptical output signal from said filter comprises a fused-fibre splitterconnected in series at its exit from the filter, having an outletconnected to an optical check receiver.

Preferably, said splitter draws less than 5% of optical power to be sentto said optical check receiver.

In particular, said optical receiver comprises a photodiode for theelectronic detection of the optical signal.

In particular, said Fabry-Perot filter has a free spectral range FSRgreater than or equal to said predetermined bandwidth.

In a fifth aspect, the present invention relates to a device formeasuring and checking the signal/noise ratio in a multi-wavelengthtelecommunication system, characterized in that it comprises:

means for extracting at least one portion of an optical signal from anoptical fibre and adapted to convey it to an optical outlet;

a tunable optical filter, connected in series to said optical outlet,adapted to produce an optical output signal in a wavelength band ofpredetermined width, comprising respective commandable actuator means;

means for receiving at least one portion of said optical outgoing signalfrom said filter;

means for commanding said actuator means of said filter, in connectionwith said receiving means, for filtering through a predeterminedwavelength band;

means for detecting the optical power passing through the filter atseveral wavelengths in said band;

means for interpolating optical power values in said band; and

comparing means for carrying out a comparison between correspondingmeans relating to said optical through-power and said opticalinterpolated power.

In particular, the means for commanding the actuators of said filtercomprises a microprocessor unit adapted to command the filter forperiodically scanning at least one portion of said wavelength band, atsteps of predetermined width.

In a particular embodiment, said actuator means for said filter arepiezoelectric actuators.

In a preferred embodiment, said tunable filter is a Fabry-Perot filter.

Preferentially, said extraction means of at least one portion of saidoptical signal comprises a fused-fibre splitter, connected in seriesalong the fibre and, more preferentially, said splitter draws less than5% of optical power.

More details will be more fully understood from the followingdescription, with reference to the accompanying drawings, in which:

FIG. 1 is a diagram of a multiwavelength telecommunication system withcheck filters according to the present invention;

FIG. 2 is a diagram of a line amplifier for use in the system of FIG. 1;

FIG. 3 is a diagrammatic section of a Fabry-Perot filter;

FIG. 4 shows the graph of the theoretical response curve of aFabry-Perot filter (optical through-intensity, on a linear andlogarithmic scale, depending on frequency);

FIG. 5 shows the diagram of the tuning circuit associated with thereceiver of FIG. 1;

FIG. 6 shows a signal spectrum detected at the preamplifier exit in thesystem of FIG. 1 (in which the first carrier does not overlie thespontaneous-emission peak);

FIG. 7 is a diagram showing the course in time of the voltages appliedto the filter and the optical power received during the channel search;

FIG. 8 shows an enlarged portion of the diagram in FIG. 7;

FIG. 9 is a qualitative diagram of the optical power measurementscarried out during the carrier-search and recognition procedure,depending on the wavelength, at the carrier of a channel;

FIG. 10 shows a diagram as in FIG. 9, at a local variation of opticalpower that does not correspond to the carrier of a channel;

FIG. 11 shows the diagram of the circuit for analyzing the signal/noiseratio at the amplifier exit;

FIG. 12 is a diagram of the spectral analyzing device on several opticallines;

FIG. 13 is a qualitative diagram of the optical power spectrum passingthrough the filter, on varying of its piloting voltage.

FIG. 14 is a diagram of a multiwavelength telecommunication systemaccording to a further embodiment of the present invention;

FIG. 15 is a graph of the passband of the fixed filters of FIG. 14.

a) Description of the System

As shown in FIG. 1, in a multi-channel optical telecommunication systemfor a wavelength-division multiplexing transmission according to thepresent invention several optical-signal sources are provided (foursources in the present example) which have wavelengths λ₁, λ₂, λ₃, λ₄included in the band of useful work of the amplifiers disposed insuccession in the system.

Said optical signals, generated separately by respective sources, arefed to a signal combiner 2 or multiplexer, adapted to send the signalsat the wavelengths λ₁, λ₂, λ₃, λ₄, simultaneously to a single opticaloutput fibre 3.

Generally, the signal combiner 2 is a passive optical device by whichthe optical signals transmitted over respective optical fibres areoverlapped in a single fibre. Devices of this kind consist for exampleof fused-fibre couplers, in planar optics, microoptics and the like.

By way of example, an appropriate combiner is combiner 1×4SMTC-0104-1550A-H put on the market by E-TEK DYNAMICS INC., 1885 LundyAve, San Jose, Calif. (U.S.A.).

Said optical signals are sent through fibre 3 to a booster 4 raising thelevel of same to a value sufficient to enable a subsequent length ofoptical fibre separating from the following amplifying means to betravelled over, while keeping a power level sufficient to ensure therequired transmissive quality.

Therefore, connected to the booster 4 is a first length 5a of opticalline usually consisting of a mono-mode optical fibre of the step-indextype, inserted in an appropriate optical cable which is some ten (orhundred) kilometers long; for example, about 100 kilometers long usingthe amplifying means described in the following and the stated powerlevels.

In some cases also optical fibres of the dispersion shifted type can beused.

At the end of said first length 5a of optical fibre, a first lineamplifier 6a is present which is adapted to receive the signalsattenuated during their travel over the fibre, and amplify them to asufficient level for feeding them to a subsequent optical fibre length5b having the same features as the preceding one and to the related lineamplifier 6b, covering the required overall transmission distance untila receiving station 7 is reached in which the signals are shared out orsplit, depending on the various transmitted channels identified by therespective wavelengths, and sent to the respective receivers 8a, 8b, 8c,8d.

The receiving station 7 comprises a pre-amplifier 9, adapted to receivethe signals and amplify them, compensating for the loss resulting fromthe subsequent demultiplexing apparatus, by providing a power levelsuitable to the sensitivity of the receiving devices.

From pre-amplifier 9 the signals are sent to a device adapted to shareout the optical signals fed to an input fibre, on several output fibres,separating them depending on the respective wavelengths. Such a device,also referred to as demultiplexer, in the present example consists of afused-fibre splitter 10 dividing the input signal into signals onseveral output fibres, four fibres in the present example, each of saidsignals being fed to a respective bandpass filter 11a, 11b, 11c, 11d,centered on each of the wavelengths of interest.

For instance, for the splitter 10 a component of a type similar to thealready described signal combiner 2 may be used, mounted in an invertedconfiguration.

In the following, bandpass filters adapted for use are described.

To the ends of the present invention and for the above described use,the booster 4 is for example an optical fibre amplifier of acommercially available type, having the following features:

input power -5 to +2 dBm

output power 13 dBm

work wavelength 1530-1560 nm.

An appropriate model is TPA/e-12, put on the market by the Applicant.

Said booster uses an erbium-doped active fibre, of the Al/Ge/Er type.

By "booster" it is intended an amplifier operating under saturationconditions, in which the output power depends on the pumping power, asdescribed in detail in the European Patent n° EP 439,867 hereinincorporated by reference.

To the ends of the present invention and for the above described use, by"pre-amplifier" it is intended an amplifier put at the end of the line,capable of increasing the signal to be fed to the receiver to a valueconveniently higher than the sensitivity threshold of the receiveritself (for example, in the case of a transmission at 2.5 Gbit/s, thepower reaching the receiver is between -26 and -11 dBm), while at thesame time introducing the lowest possible noise and maintaining thesignal equalization.

In the described experiment, for producing the pre-amplifier 9, aone-stage amplifier using the same active fibre as used in amplifiers6a-6b described in the following was employed and it was mounted in aco-propagating configuration. For particular embodiments, apre-amplifier expressly designed for the purpose can be adopted.

The configuration of the above described transmission system isparticularly appropriate to provide the desired performance, especiallyfor wavelength-multiplexing transmission over several channels, in thepresence of a particular selection of the properties of the lineamplifiers being a part thereof, especially with regard to thecapability of transmitting the selected wavelengths without some of thembeing penalized or attenuated with respect to others.

In particular, a uniform behavior for all channels can be ensured, in awavelength band included between 1530 and 1560 nm, in the presence ofamplifiers adapted to operate in cascade, making use of line amplifierscapable of giving a substantially uniform (or "flat") response at thedifferent wavelengths, when operating in cascade.

b) Line Amplifier

An amplifier intended for the above purpose and provided for use as aline amplifier can be made according to the diagram shown in FIG. 2 andit comprises one erbium-doped active fibre 12 and a respective pumplaser 13, connected thereto by a dichroic coupler 14. One opticalisolator 15 is located upstream of the fibre 12, in the travel directionof the signal to be amplified, whereas a second optoisolator 16 islocated downstream of the active fibre itself.

The amplifier further comprises a second erbium-doped active fibre 17associated with a respective pump laser 18 by means of a dichroiccoupler 19, also connected for countercurrent pumping in the exampleshown. Downstream of the fibre 17 another optical isolator 20 istherefore present.

The pump lasers 13, 18 preferably are lasers of the Quantum Well typeand have the following features:

emission wavelength λ_(p) =980 nm;

maximum optical output power P_(u) =80 mW

Lasers of the above type are produced for example by: LASERTRON INC., 37North Avenue, Burlington, Mass. (U.S.A.).

The dichroic couplers 14, 19 in the example are fused-fibre couplers,formed of mono-mode fibres at 980 nm and in the 1530-1560 nm wavelengthband, with variations <0.2 dB in the output optical power depending onpolarization.

Dichroic couplers of the above type are known and commercially availableand produced for example by GOULD Inc., Fibre Optic Division, BaymeadowDrive, Glem Bumie, Md. (U.S.A.), and by SIFAM Ltd., Fibre OpticDivision, Woodland Road Torquay Devon (GB).

The optical isolators 15, 16, 20 are optical isolators of a typeindependent of the transmission signal polarization, and have anisolation greater than 35 dB and a reflectivity lower than -50 dB.

The isolators herein used are model MDLI-15 PIPT-A S/N 1016 availablefrom ISOWAVE, 64 Harding Avenue, Dover, N.J., U.S.A.

In the described systems the line amplifiers are provided for operationwith an optical overall output power of about 14 dBm, with a gain ofabout 30 dB.

In the above described amplifiers an erbium-doped active fibre has beenused, as described in detail in the Italian Patent application N°M194A000712 of Apr. 14, 1994 filed in the name of the assignee of thisapplication which is herein incorporated by reference, and the contentsof which are hereinafter summarized.

The composition and optical features of the fibre used are summarized inthe following Table 1.

                                      TABLE 1                                     __________________________________________________________________________    Al.sub.2 O.sub.3                                                                         GeO.sub.2                                                                            La.sub.2 O.sub.3                                                                     Er.sub.2 O.sub.3                                                                        λ.sub.c                             FIBRE                                                                             % p                                                                              (% mol)                                                                           % p                                                                              (% mol)                                                                           % p                                                                              (% mol)                                                                           % p                                                                              (% mol)                                                                           NA nm                                         __________________________________________________________________________    A   4  (2.6)                                                                             18 (11.4)                                                                            1  (0.2)                                                                             0.2                                                                              (0.03)                                                                            0.219                                                                            911                                        __________________________________________________________________________     % p = percent content by weight of oxide in the core (average)                % mol = percent content by moles of oxide in the core (average)               NA = Numerical Aperture (n1.sup.2 -n2.sup.2)1/2                               λ.sub.c = cutoff wavelength (LP11 cutoff)                         

Tests on the compositions were carried out on a preform (before spinningof the fibre) by a micro-probe combined with a scanning electronmicroscope (SEM Hitach).

Tests were conducted at 1300 times magnification on discrete pointsdisposed along a diameter and separated by 200 mm one from the other.

The fibre was made by the vacuum plating technique within a tube ofquartz glass.

The incorporation of germanium as the dopant in the SiO₂ matrix in thefibre core was obtained during the synthesis step.

The incorporation of erbium, alumina, and lanthanum into the fibre corewas obtained by the so-called "solution doping" technique, in which awater solution of the dopant chlorides is brought into contact with thesynthesis material of the fibre core, while this material is in theparticulate state, before consolidation of the preform.

More details on the solution doping technique can be found for examplein U.S. Pat. No. 5,282,079, which is herein incorporated by reference.

The first active fibre 12 was approximately 8 m long. The second activefibre 17 was about 11 m long.

c) Transmission Experiment

The described configuration is particularly adapted for, and givessatisfactory results in transmissions over distances on the order of 500km, at a high transmission speed, for example 2.5 Gbit/s (therebyobtaining, with four multiplexed wavelengths, a transmission capabilitycorresponding to 10 Gbit/s on a single wavelength), making use of fourline amplifiers, one booster and one pre-amplifier.

With the above described configuration a high-speed transmission onseveral channels was achieved.

The signals used for the transmission experiment on several channelswere respectively generated by a laser DFB at 1534 nm, modulated with anexternal modulator at 2.5 Gbit/s; a continuous-emission laser DFB at1556 nm, produced by ANRITSU; a continuous-emission laser DFB at 1550nm, produced by ANRITSU; a continuous-emission laser ECL of a variablewavelength preselected at 1544 nm, model HP81678A, produced by HEWLETTPACKARD COMPANY, Rockwell, Md. (U.S.A.).

The external modulator employed for the modulation at 2.5 Gbit/sconsisted of a modulator of the Mach-Zender type in LiNbO₃, produced bythe assignee of this application, model MZM 15301.

In the experiment carried out each of the filters 11 was provided with arespective tuning device, while the spectrum analyzing device wasapplied in succession to the outlets of the line amplifiers, at pointsshown in FIG. 1, as described in the following.

The overall optical power at the pre-amplifier 9 entrance was -20 dBm.

d) Selection of the Channel

In order to send the respective channel to each receiver 8a-8d, thesplitter 10 shares out or splits the whole optical signal received onthe respective outlets and each filter 11 lets only the signal ofinterest pass on the respective output fibre 21a, 21b, 21c, 21d,carrying out an optical filtering in a narrow band about the respectivesignal-carrying wavelength.

To this end, filters 11 preferably consist each of a filter of theFabry-Perot type, for example of the type previously specified, providedwith a tuning device 22 of the piezoelectric type. Alternatively, formaking filters 11, also acousto-optical filters can be used as well astunable gratings or thin-film interference filters tunable by means oftuning devices of the electromechanical type.

In all the above cases, and possibly in other unlisted similar cases,commandable actuator devices are at all events present which act on thefilter, and through which the selection of a desired passband isoperated.

While for the sake of simplicity the tuning device 22 has been shownonly in combination with one of the filters 11. In the describedexperiment each filter 11 was provided with a respective tuning device.

A Fabry-Perot filter with a tuning device of the piezoelectric type,adapted for the above stated use is for example model FFP-TF,commercialized by MICRON-OPTICS, INC., 2801 Buford Hwy. Suite 140,Atlanta, Ga., U.S.A., or model MF 200 commercialized by QUEENSGATEINSTRUMENTS Ltd., Silkwood Park, Ascot, Berkshire SL5 7PW, GB.

A diagram showing the structure of the Fabry-Perot filter is reproduced,just as an indication, in FIG. 3 and its response spectrum isrepresented in FIG. 4, in a linear diagram on the left and a logarithmicdiagram on the right.

As shown in FIG. 3, the filter comprises respective ferrules 26, intowhich the ends of respective optical fibres 27 are housed. Reflectiveelements 28 facing each other form a cavity the width of whichdetermines the filter through wavelengths.

Given the periodic nature of the phenomenon, the filter has severalthrough-wavelengths separated from each other, in the frequency domain,by regular intervals the width of which is usually indicated as FreeSpectral Range, FSR, as shown in the graph in FIG. 4.

Ferrules 26 are in turn housed within respective supports 29 and thepiezoelectric actuators 30 are seated between the opposite supports 29.

A voltage applied to the piezoelectric actuators 30 modifies the size ofsame and thus modifies the cavity length l_(c) between the reflectiveelements, thereby enabling tuning of the filter in the desiredwavelength band, in response to an appropriate command signal appliedthereto by an associated tuning device.

An ideal Fabry-Perot filter has a general equation of the transmissioncoefficient depending on the wavelength λ (Airy function): ##EQU1##wherein: n is the travel difference between the interfering beams,expressed in wavelength units,

R is the reflection intensity coefficient,

NR is the reflection "finess" of the cavity, defined as:

    NR=FSR/δ

wherein

δ is the bandwidth at half power (transmission point at 3 dB) of thepassband, and

FSR=λ/n.

As shown in FIG. 4, the Airy function is a periodic function withtransmission peaks at consecutive whole values of n.

Preliminarily, each filter is characterized in order to identify thepreliminary parameters, specific for the filter in use, that will beemployed for the tuning operations described in the following, and inparticular the voltage gradient G which is to be applied in order toobtain a given variation speed in the transmitted wavelength (forexample 0.2 nm every 0.5 ms).

Since the process of the invention is capable of tuning a filter andkeeping it tuned on a particular carrier, it is first of all describedhereinafter the tuning device and subsequently the operating method.

d1) Tuning Device

The tuning device 22 is adapted to compensate for slips of theFabry-Perot filter, of thermal nature for example, and hysteresis of therelated actuators, and to keep it "hooked" at the wavelength of therespective signal.

The diagram of the tuning device is detailed in FIG. 5, in whichconnections of the optical type are shown in solid line and electricconnections in dotted line.

The tuning device 22 comprises an optical coupler 23 located downstreamof filter 11, adapted to draw or extract a portion of the optical signalfrom the fibre 21 directed to the respective receiver, a detector 24adapted to convert the received optical signal to an electronic form andan analysis and check circuit 25 adapted to generate the electricpiloting signal of the respective filter 11.

In more detail, the detector 24 comprises a photodiode 31 connected toan electronic amplifier 32. The output of the amplifier 32 is sent to ananalog/digital converter 33, and from the latter to a microprocessor 34.In turn, the microprocessor output is sent to a digital/analog converter35, the signal of which pilots the filter 11.

The optical coupler 23 preferably is a fused-fibre coupler 95/5, adaptedto draw 5% of the optical power. An appropriate coupler is modelSWBC2PR3PP210, produced by E-TEX DYNAMICS INC., 1885 Lundy Ave, SanJose, Calif. (U.S.A.).

The photodiode 31 is a photodiode PIN, in InGaAs; for example, modelETX75 FJ SLR, available from EPITAXX OPTOELECTRONICS DEVICES, 7 GraphicsDrive, West Trenton, N.J., U.S.A., or model FD100F, available fromFERMIONICS OPTOTECHNOLOGY, 4555 Runway Street, Simi Valley, Calif.,U.S.A.

The electronic amplifier 32 preferably consists of an amplifier having avery small offset, for example model LTC 1051 available from LINEARTECHNOLOGY CORPORATION, 1630 McCarthy Blvd, Milpitas, Calif., U.S.A.

The analog/digital converter 33 is a 12-bit converter, commercialized bysaid LINEAR TECHNOLOGY.

The digital/analog converter 35 is a 12-bit converter, commercialized byMAXIM INTEGRATED PRODUCT, 21C Horseshoe Park, Pangboume Reading, UK.

The microprocessor 34 is a 8-bit microprocessor model ST90E40, marketedby SGS THOMSON (viale Milanofiori, Strada 4, Palazzo A4, Assago, MI,IT).

The microprocessor adapted for use in the present invention is a devicecomprising processing units, memory registers and the like, known per seand commercially available, and therefore not further described.

Conveniently, the microprocessor 34 is provided with another output sentto a system alarm unit 36 capable of supplying alarm signals, forexample relating to the carrier hooking and the presence of opticalpower on the detector 24, and transmitting a check signal to an opticalchange-over switch 37 adapted to open the optical circuit to thereceiver 8, disconnecting it from reception, if hooking to the carrierof the channel related thereto did not occur within a predeterminedtime, 40 ms for example, and keeping said circuit disconnected over thetime necessary to restore said hooking.

An appropriate optical change-over switch is for example modelSW11Z4-00NC, available from JDS FITEL INC., 570 Hest on Drive, Nepean,Ontario, CA (U.S.A.).

d2) Tuning Method

The above described tuning device operates through a procedure involvingthe following steps: initial setting of the filter to identify thewavelength range in which all searched carriers are located, division ofthis range to identify the wavelength range in which each carrier islocated, scanning of the range in which a single carrier is to besearched, recognition of the searched carrier and finally, hooking andhooking-holding to keep the filter centered on the carrier itself.

The present invention is particularly useful in the case in which therelation between voltage and central wavelength of the filter passbandsuffers from hysteresis and memory phenomena, or similar phenomenaaffecting the stability in time of the through-wavelength or therepeatability of same, as for example in the case herein described offilters having piezoelectric actuators.

According to the invention, it has been noted that in order tocompensate for said hysteresis, memory and similar phenomena, it isparticularly convenient to activate the piezoelectric actuators acertain number of times before making the desired correlation betweenthe piloting voltage and the corresponding wavelength, always applyingthe same voltage gradient starting from the same initial voltage.

In the following the voltage gradient denoted by G has been used, whichhas been determined for each filter type or model during the preliminarysetting step.

Said gradient is defined as the voltage ramp that, applied to thefilter, generates a predetermined variation in time at the centralwavelength of the filter passband (for example 0.2 nm every 0.5 ms)under environmental laboratory conditions.

For example, such a ramp can consist of discrete increments of 0.02 mVevery 0.05 ms.

In a preferred embodiment, in the case of four spatial carriers withintervals included between 4 and 8 nm described by way of example, thesystem can operate in the following manner.

d2.1) Setting

On turning the system on, the microprocessor initially carries outsetting of the filter, with reference to the spontaneous emissionpresent in the optical signal following the action of the amplifyingfibres of the amplifiers interposed along the line. This spontaneousemission is characterized by a sure and stable spectral reference,represented by the wavelength corresponding to its peak.

The spontaneous-emission profile overlapped with the peaks of thetransmitted signals is shown in FIG. 6 representing the optical spectrumdetected at the pre-amplifier 9 outlet for one of the branches separatedby the splitter 10.

For the described procedure, the filter is selected with a free spectralrange (FSR, measured by application of a voltage ramp of gradient G)which is greater than or equal to the width of the spontaneous-emissionband (that is the wavelength band in which there is a clearlydistinguishable spontaneous emission, for example 3 dB higher than thebackground noise).

In the experiments relating to the described example, filters having a(nominal) FSR of 45 and 60 nm were used.

To carry out setting, as shown in FIG. 7, the microprocessor 34 commandsthe application of a first voltage V₀ to the filter, which isintermediate between the minimum and maximum piloting voltage of thefilter (usually such minimum and maximum voltages are 0 and 45 V,respectively).

In addition, the filter is selected in such a manner that, on varying ofthe voltage (with a gradient G) between the minimum and maximum values,an excursion of the intermediate wavelength of the passband equals atleast 3 FSR.

If in the subsequent step decreasing voltages are used, said voltage V₀is higher than the average voltage (22 V) by a given value calculated byadding to the average voltage value such a voltage increment that thescanned range will be centered within the filter work range.

For example, this increment can be half the voltage variationcorresponding to the excursion of the intermediate wavelength of thepassband equal to a whole FSR, multiplied by a factor greater than 1(1.5 for example).

The choice of using increasing or decreasing voltages during the rangescanning step is done preliminarily. Scanning with voltage ramps havingopposite gradients relative to those shown in the example described inthe following are deemed to give similar results.

In the following of the present description reference is made to thecase in which decreasing wavelengths correspond to decreasing voltages.The opposite case can be dealt with in the same manner by suitablyexchanging "decreasing" with "increasing".

d2.1i.

Starting from voltage V₀ the microprocessor carries out a scanning ofthe optical band, causing decreasing, with gradient G, of the voltageapplied to the actuators until a minimum given value, for exampleidentified as lower than voltage V₀ by a value equal to the voltagevariation value corresponding to one FSR multiplied by the same factoras previously pointed out, which is higher than 1.

During this voltage excursion, the voltage applied to the filter isregistered, as well as the corresponding optical power received by thedetector 24 at each predetermined time interval (0.5 ms, for example)and the power value of each measurement is compared with the precedingone, until an absolute minimum of optical power received P_(min) isfound along with the voltage V_(min) corresponding thereto.

If an optical power minimum cannot be identified, for example in theabsence of signals and spontaneous emission, as shown in the first threevoltage ramps in FIG. 7, after scanning of the whole range the systemgoes back to value V₀, by applying a voltage step, and the cycle isrepeated.

In the presence of a spontaneous emission (and signals), given theperiodicity of the filter passband, the profile of the received power onvarying of the position of the reflective elements in the filter (thatis the piloting voltage of the piezoelectric actuators, although thecorrelation between voltage and wavelength has been found non-linear asit is subjected to hysteresis and thermal slip phenomena) is repeated asqualitatively shown in FIG. 13. In fact, when the excursion of themovable reflecting elements in the filter corresponds to a multipleinteger of FSR, a new transmissive window comes into register with theemission band of the fibre, so that the detector 24 registers thespontaneous-emission spectrum again (and the carriers present therein,if any).

As shown in FIG. 13, said minimum power value P_(min) is close to theregion included between two spontaneous-emission figures.

When a minimum value of optical power has been identified, scanning ofthe whole range is repeated a number of times (three times in theexample shown in FIG. 7), still applying the voltage gradient G, untilsaid minimum substantially occurs at the same voltage value, which isdenoted as V_(min), for example within an interval of ±1 V.

d2.1.ii. When a stable value of V_(min) has been identified, within theabove interval, the microprocessor commands the application of voltageV_(min) to the filter, which stably corresponds to the detected valueP_(min) and, starting from this voltage value, the application ofsuccessively increasing voltages, with the predetermined mean gradientG.

During this voltage ramp, the voltage value V₁ is registered, whichvalue corresponds to the maximum of the spontaneous emission, or thepeak of the first carrier encountered (at 1534/1536 nm, for example), ifthe maximum of the spontaneous emission is not detected. Thespontaneous-emission maximum is the main spectral reference searched forwith the present procedure. However, if this maximum is overlaid by thefirst carrier, sometimes it could be not detected. In this case howeverthe first carrier of known wavelength is present and it is taken as thespectral reference.

The distinction between the spontaneous-emission maximum and the peakcorresponding to a carrier is carried out by a recognition procedure,active during the whole cycle and described in the following.

The spontaneous-mission maximum is identified as such at a sampling, ifit is a maximum value that is not recognized as corresponding to acarrier, based on said recognition procedure, and if the value of themeasured power keeps lower than said maximum value over a predeterminednumber of successive samplings (9 for example). If nospontaneous-emission maximum is identified, the first carrierencountered is taken as the carrier overlying the spontaneous-emissionmaximum, that is, in the present example, the carrier at 1534/1536 nm.

The voltage ramp is stopped at a value V₂, after an increment ΔV₁, withrespect to voltages corresponding to the spontaneous-emission maximum(or the first carrier), enabling the region of the upper extreme of thespontaneous emission (1570 nm) to be reached without however reachingthe spectrum corresponding to the next transmissive window of thefilter. Said value ΔV₁ is different depending on whether V₁ wasidentified as corresponding to the spontaneous-emission maximum or thefirst carrier. This difference is equal to the required voltageincrement, applied with gradient G, to move the transmitted wavelengthfrom the value corresponding to the spontaneous-emission peak to thevalue corresponding to the first carrier.

By way of example, if V1 was identified as the spontaneous-emissionpeak, the overall voltage increment ΔV₁, in the present example, isabout 10 V.

Since in the subsequent steps the starting point for applying thevoltage gradient is different (see the above considerations concerningthe hysteresis and memory phenomena of piezoelectric actuators),different wavelength values are obtained, the voltage increment appliedwith gradient G with respect to the voltage corresponding to thespontaneous-emission maximum or the first carrier being equal. The abovemust be taken into account in determining the values of ΔV₁ and inselecting the width of a filter type having an appropriate FSR width, soas to cause the arrival point to be always beyond the last carrierprovided in the operating use but before the subsequentspontaneous-emission spectrum which is encountered, due to the filterperiodicity.

d2.1.iii. A quick (stepped) voltage decreasing ΔV₂, of predeterminedvalue, is applied starting from value V₂, this decreasing preferentiallycorresponding to approximately 1.5 the preceding increment ΔV₁. Thisdecreasing also depends on gradient G according to which the filtercommanding voltage is applied.

The applied voltage decreasing value ΔV₂ is selected in order toposition the filter such that the reached point V₄ corresponds to awavelength close to the minimum value P_(min).

In the absence of hysteresis phenomena in the piezoelectric actuators ofthe filter, in the described example, step iii. would lead to awavelength lower than that corresponding to point P_(min). It has beenhowever experimentally contemplated that this decreasing, in thedescribed example, during the first cycles in which it is applied doesnot enable said minimum point of the spontaneous emission to be reachedand that sometimes does not even enable the spontaneous-emission peak tobe overcome.

d2.1.iv. Starting from point V₄ the search of the minimum is repeated,by decreasing the applied voltage, with the same gradient G, until apower threshold value is reached, consisting for example of thepreviously measured P_(min) value increased by 3 dB. The voltagecorresponding to this threshold is registered as the new V_(min) value.

d2.1.v. The preceding steps ii iii iv are repeated a predeterminednumber of times, five times for example. During these cycles astabilization of the V_(min), V₂ and ΔV₂ values successively foundprogressively occurs, due to the progressive reduction of the effectsresulting from the memory phenomena of the piezoelectric materials,thanks to the repeated application of a periodic course of the guidevoltage.

The repetition number is selected such that the spontaneous-emissionpeak (or the first carrier, or, more generally, a known and preselectedreference in the spectrum) occurs at a substantially constant voltage,wherein accuracy is equal to the voltage variation corresponding to asampling interval (200 mV, for example).

d2.1.vi. Subsequently, other cycles (5 for example) are carried out andthey differ from the preceding ones in that, after the application ofthe voltage increment ΔV₂, the search for the minimum according to theprocedure described at point i. is not carried out, but the voltageincrease is directly executed, with gradient G. This enables thedetection of the maximum of the spontaneous-emission peak or the firstcarrier to be carried out through the application of voltage cyclessimilar to each other.

d2.1.vii. Identification of the Carrier

The above cycles (point d2.1.vi) being completed, the microprocessorregisters the voltage value corresponding to the spontaneous-emissionpeak (or the first carrier) and those corresponding to the extremes ofthe intervals within which the different carriers are.

These intervals are defined so as to contain the desired carrier alone(although in the presence of tolerances and possible residual hysteresisand shift phenomena that cannot be compesated for).

d2.1.viii. Carrier Hooking

Once the carrier is identified, as described in the following (pointd2.3) and the power value corresponding to its registered maximum, themicroprocessor applies a negative voltage step ΔV₃ of a value determinedexperimentally, typically of 1-2 V, capable of reducing or eliminatingthe inertia in the filter motion.

Then a decreasing voltage is applied, with a gradient of reduced(halved, for example) value, with respect to gradient G hitherto used,and the optical power values corresponding to the applied voltage valuesare detected at each predetermined time interval (consisting for exampleof 0.5 ms).

After overcoming in succession first a voltage value corresponding to apower equal to 90% of the maximum value detected on recognition of thecarrier and afterwards a voltage value equal to 50% of the maximum valueitself, scanning is stopped and repeated in the opposite way (that is,for example, with an increasing voltage gradient if previously adecreasing voltage gradient was used), with a gradient of a furtherreduced value by a given amount, for example again halved.

The scanning procedure is repeated until a voltage gradient is appliedwhich enables the voltage increment at each step to correspond to asufficiently small wavelength variation, of about 0.02 nm for example.

Then two other scannings are carried out with this gradient and thevoltage corresponding to the intermediate value between the twolast-measured values corresponding to 50% of the maximum value isidentified as the voltage corresponding to the carrier position.

d2.1.ix. Carrier Selection

The detected reference values (minimum-power voltage and voltagecorresponding to the spontaneous-emission peak, first carrier or otherknown reference) constitute a new setting reference for the filter,based on which the microprocessor can modify the setting relation firstused for calculating the voltage intervals within which the carriers areto be searched out (and in particular the one corresponding to therelated filter).

Cycles pointed out at point d2.1.vi being completed, the microprocessorcarries out the carrier recognition by adopting the method hereinafterindicated (point d2.2). If this recognition takes place within thevoltage interval corresponding to the searched carrier (point d2.1.vii),the hooking procedure is activated (point d2.1.viii) and thehooking-holding procedure is activated (point d2.3).

d2.2. Carrier Recognition

Power measurements of the optical signal downstream of the filter arecarried out at each predetermined time interval (0.5 ms for example).

Measurements thus made fill a cyclic memory, a 15-value memory forexample. On storing a new value, the value which has been stored for thelongest period of time is discarded.

After every detection, measurements in the memory are divided into threegroups, as diagrammatically shown in FIG. 9 and sequentially denoted byI, II, III, wherein groups I and III, called end groups, contain thesame number of measurements, four for example, and the intermediategroup II contains the intermediate measurements executed, preferably oddin number, seven for example.

By discarding the measurements of the intermediate group, themeasurements of the end groups I and III are interpolated with anappropriate algorithm and the interpolating curve is extended also inthe intermediate region, the measurements of which have been discarded.

This interpolation gives origin to a curve R, shown in phantom in FIG.10.

Based on this interpolation curve, the microprocessor checks whether themeasurement carried out at the central wavelength λ_(c) in theintermediate subgroup II has the measured optical power P_(m) greaterthan the optical power P_(i) calculated through the interpolationfunction R at the same wavelength, by a predetermined factor, 3 forexample. If this condition is complied with, the wavelength λ_(c) isidentified as the wavelength of the optical carrier.

The condition corresponding to the desired ratio is, by way of example,graphically represented in FIG. 9, in which the ratio P_(m) /P_(i) meetsthe indicated prescription.

d2.3. Hooking-holding

Two values of optical threshold power are calculated, respectivelyreferred to as "attention threshold" P_(A) and "release threshold"P_(S). These values are calculated for example as 80% and 25%respectively of the maximum power value of the carrier, detected duringthe hooking step.

The carrier of the desired channel being hooked, the microprocessor 34by a stepped application commands the required voltage variation forpositioning of the passband of filter 11 at the value detected as thecentral value of the concerned carrier.

Starting from this position, the passband of the filter is shifted atregular intervals, by a step of a given value (corresponding for exampleto 0.02 nm, every n seconds) in one direction, by applying acorresponding (positive or negative) voltage increment to the filteractuators.

At each step, several samplings (8 for example) of the received powerare executed in a quick succession (one every 0.5 ms, for example) andthe average of the detected values (P_(B)) is calculated.

If the average is higher than the previously measured average value,another step is carried out going in the same direction, the newsamplings being executed and the new average (P_(B)) being calculated.

For the first step, value (P_(B)) is compared with the maximum carriervalue registered during the hooking step.

If the average (P_(B)) is lower than the preceding one, the shiftingdirection is reversed, two steps are carried out and samplings of thepower are executed at the second step.

If the measured power average remains higher than (P_(A)), after apredetermined period of time (16 s, for example), the number ofsamplings executed at each step is doubled, until a predeterminedmaximum number is reached, 1024 for example, which corresponds, with aninterval of 0.5 ms between each measurement, to an overall sampling timeof 512 ms.

Conveniently, if the average power measured at one step goes down undersaid attention threshold (P_(A)), the number of samplings executed ateach step is reduced (to 8, for example), in order to allow a quickerpursuit of the maximum, although under conditions of greater noise inthe system.

The above described hooking procedure enables the system operation to befollowed continuously, compensating for possible slips, both of thesignal and filter, due for example to temperature ranges, vibrations,disturbance or other.

If the measured instantaneous power goes down below (P_(B)) and stays tothis value over a certain period of time (40 ms, for example), thefilter "is released" and it is necessary to start again from the Settingprocedure (see point d2.1).

Conveniently, values P_(S) and P_(A) can be periodically updated, usinga long-period average value determined based on the detected valuesP_(B), so as to conform the procedure to the variations in time of thesystem performance as a whole.

e.) Optical-spectrum Analysis and Determination of the Signal/noiseRatio

A device according to the present invention is conveniently applied tothe analysis of the optical spectrum as well, in order to identify thecarriers actually present and determine their signal/noise ratio alongthe line.

For the purpose, as shown in FIG. 1 and, in more detail, in FIG. 11, inwhich connections of the optical type are shown in solid line andelectric connections are shown in dotted line, an optical coupler 38having the same features as coupler 23 previously described is locatedat the outlet of an amplifier, or each amplifier, or a pre-amplifier 4,6, 9.

Through the coupler the signal fraction drawn is sent to a filter 39,and from the filter to an optical detector 40, adapted to convert thereceived optical signal to an electronic form, and an analysis and checkcircuit 41, adapted to generate the electric piloting signal of filter39.

In greater detail, the detector 40 comprises a photodiode 42, connectedto an electronic amplifier 43. The amplifier 43 output is sent to ananalog/digital converter 44, and from the latter to a microprocessor 45.In turn, the microprocessor output is sent to a digital/analog converter46, the signal of which pilots the filter 39 through the digital/analogconverter 46.

Conveniently, the microprocessor 45 is provided with another output sentto a system alarm unit 47, capable of providing the desired alarmsignals, in the same manner as described with reference to filters 11.

The components used for the optical spectrum analysis can be the same aspreviously used for tuning of filter 11.

The described device operates as follows.

The microprocessor 45 commands the execution of a setting operation and,after identification of the region where the carriers are, recognitionof the carriers as previously described is carried out. For determiningthe signal/noise ratio, for the measurement group which has beenrecognized as corresponding to the carrier of the searched channel, themicroprocessor calculates the interpolation curve S (by extending itthrough the subgroup II) of the measurements of the end subgroups I andIII and calculates the value of the interpolation function at theintermediate point of Group II.

Conveniently, these operations are carried out with steps correspondingto a 0.2 nm wavelength, at intervals of 0.5 ms.

For determining the signal/noise ratio, the microprocessor establishesthe ratio between the signal power value optimized according to theprocedure at point d.2.3 and the calculated value of the interpolationfunction.

This ratio constitutes the searched signal/noise ratio for the channelrelating to the considered carrier and can be used by the microprocessor45 itself or the alarm unit 47, or another apparatus connected thereto,to carry out system checks and the like.

The procedure can be repeated for each of the transmitted channels bytuning the filter in succession on each of them, then drawing theinformation concerning operation of the whole system, continuouslyduring operation of same.

In another embodiment, shown in FIG. 12, a single filter 48 can beassociated, through an optical change-over switch 49 adapted to beselectively connected to one of fibres 50, with several optical lines51a, 51b, 51c, 51d, 51e, 51f and so on, for example at the respectiveoptical amplifiers 52a-f, from the respective outlets of which a signalfraction (5% of the optical power, for example) is extracted, throughthe respective couplers 53. The filter 48, in the same manner as abovedescribed, is associated with a respective optical detector 54 and ananalysis and check circuit 55.

In this manner, by a connection between the change over switch 49 andfibres 50, drivingly carried out for a particular line, or carried outsequentially based on a previously constituted procedure, all theoptical lines 51 and the respective amplifiers can be checked by asingle unit.

By the system of the invention it is possible to achieve hooking of thefilter to the previously established carrier in a period of time of100-300 ms.

In the case of four channels, that is four carriers at wavelengthsspaced apart some nanometers (4-8 nm, for example) from each other, theabove described search and hooking procedure has proved to be efficientin order to achieve hooking to the desired carrier for each of thetransmitted channels alone, as a stable tuning-holding on the carrieritself.

When more than four channels are transmitted with the related carriersto a shorter distance, tuning and hooking may require closer tolerancesfor the localization of the search bands for each filter. To this end,the setting range and the search and fine-tuning bands can beconveniently defined by making use of other references at the knownwavelength, in addition to the voltage value corresponding to the use ofthe spontaneous-emission peak or the signal peak, as above described.

It should be noted that, in the absence of the searched carrier, forexample when the related channel is turned off, the procedure ismaintained active until the related channel is activated again.

In one aspect, the method of the present invention concerns therecognition of optical signals through the analysis of the opticalspectrum containing said signals, based on the profile of the spectrumitself.

In addition, the analysis of the optical spectrum enables particularfeatures of the signal under examination to be determined, such as forexample the signal/noise ratio.

In another aspect, the method according to the invention relates to theidentification and filtering of optical carriers through the analysis ofthe optical spectrum containing said carriers and the recognition of thecarriers based on the profile of the spectrum itself.

In a particular embodiment, in the method of searching and hooking thecarriers in a multi-wavelength optical telecommunication systemaccording to the present invention, it is provided that the tunablefilter be repeatedly operated through its adjustment band, by applying,with a periodic course, a piloting voltage varying between identifiedand repeatedly updated extremes in connection with optical-powerreference values known as stably corresponding to known wavelengths.

In a particular aspect of the invention, voltage is applied according tocycles becoming increasingly more similar to themselves in order toeliminate ambiguities caused by the hysteresis of the actuators used foroperation and tuning of the filter. The piloting voltage values areidentified and repeatedly updated in relation to recognition in theoptical spectrum of known references, stably corresponding to precisewavelength values.

In a preferred case, convenient values of spectral references comprisethe spontaneous-emission peak of erbium of the optical amplifierspresent along the line and, possibly, one or more carriers known assuch.

Based on said references, in addition to the initial setting relation ofthe filter, scanning of the voltage range is commanded with a variationlaw of the periodic voltage at constant gradients, until a voltage rangeis identified which stably corresponds to the predetermined wavelengthrange for the channel search.

Said voltage range is therefore divided into bands, within which each ofsaid searched channels is provided to be located.

Within a selected band corresponding to the channel searched out by thespecific filter, the carrier is identified by means of a recognitionprocedure consisting in comparing the features of an optical-powermaximum value with the corresponding interpolated features based on thevalues measured at contiguous voltages.

Said procedure therefore, even in the presence of a filter subjected toslips and having high sensitivity to the stress speed of the relatedactuators, enables the search range of each carrier to be determined andthe carrier itself to be identified.

In addition, the apparatus itself and the carried out measurementsenable a continuous determination of the signal/noise ratio to beobtained for each of the concerned channels.

While the present invention has been described with reference to theemission features typical of a connection making use of opticalamplifiers with a particular active fibre, being in the presence ofdifferent emission spectra and/or different signal distributions orfeatures, or filters having particular response features, a personskilled in the art will be able to determine, based on the teachings ofthe present invention, the reference features and values of the signaland the scanning cycles of the optical band, based on which tuning ofthe specific system is accomplished.

In a more general form, the present invention applies to each system inwhich a response signal depends on an adjustment signal subjected tohysteresis or disturbance the amount of which cannot be predetermined,in which it is possible to identify at least two reference signals ofknown value, adapted to constitute the base for the range scanning.

In a further embodiment, as shown in FIG. 14, fixed filters 56a, 56b,56c, 56d are located before each tunable bandpass filter 11a, 11b, 11c,11d.

Such fixed filters 56a, 56b, 56c, 56d have a respective bandpasssufficiently broad, such as to include both the tolerance of the centerwavelength of the relevant one of the emitters 1a, 1b, 1c, 1d, and thewavelength shift of the relevant tunable bandpass filter 11a, 11b, 11c,11d, but at the same time sufficiently narrow to reduce the level of theother signals and of the spontaneous emission peak below a predeterminedamount.

For example, a suitable pass band of the fixed filter (in a 4 wavelengthsystem, as described) is about 2 nm. The predetermined amount by whichthe non-selected wavelengths are to be reduced is preferably at least 20db, and, more preferably, at least 25 dB.

Suitable fixed filters are known and marketed, for example, by E-TEK.

The passband of the fixed filters is shown in FIG. 15.

As an alternative, a similar result can be obtained by using awavelength selective demultiplexer to replace the splitter 10.

A low frequency tone (preferably between 30 and 200 Khz) is superimposedon the modulation of the optical-signal sources 1a, 1b, 1c, 1d.Preferably, each source has a tone with a frequency different from theothers.

The tunable bandpass filters 11a, 11b, 11c, 11d are located between therelevant fixed filter and receiver 8.

Each tunable filter performs the search of the signal by searching amaximum of the optical power, according to the technique describedabove.

When the maximum of the optical power has been detected, the analysisand check circuit 25, associated to the detector 24, checks for thepresence of the modulation tone associated with the relevant signalsource. When such tone is present, the signal is recognized as thedesired channel and the filter is maintained locked on the same signal,with the technique described before.

Being the tone missing, the unit 25 detects a failure in the transmitteror in the line, and, optionally, an alarm signal may be generated.

It must be observed that in an amplified system, the tones superimposedonto the signals cannot be used to distinguish the optical sources,because the presence of the tones causes a modulation of the spontaneousemission (i.e. in the whole transmisison band).

Because of the presence of the optical amplifiers along the line, incorrespondence with a given signal at a given wavelength, all the tonesof all the sources are detected, with an intensity which can even begreater than that of the given signal itself.

According to the invention, the tone is used to be informed of thepresence of the searched signal in the already selected peak (the toneat a given frequency being absent in case the relevant optical source isoff), but it is not used to select the signal itself.

In this embodiment of the invention, the selection of the signal searchrange in which the signal can be individually present is obtained byusing the fixed filter before the tunable band-pass fitter, and the goodsignal/noise ratio thus obtained downstream the filters (at least 10 dB)allows to identify the signal when it is higher in intensity by a givenamount over the spontaneous emission.

For example, in a system according to the invention the spontaneousemission (as detected downstream the filters) is lower than -25 dBm,while the signal may range between -7 and -18 dBm.

The steps to maintain the tunable filter in correspondence of the signalare made in the same manner as described before.

According to an aspect, the present invention allows, separately, to besure that the desired signal is present, by detecting the relevant tone,and to be sure that the detected signal is the desired one, by using thefixed filter.

We claim:
 1. An optical telecommunication method comprising the stepsof:generating at least one optical transmission signal, at apredetermined wavelength included in a predetermined wavelength band;transmitting said optical transmission signal through an optical fiberto a receiving station, comprising at least a receiving unit; feeding anoptical signal comprising said optical transmission signal to arespective one of the receiving units in said receiving station, througha passband filter, wherein said optical signal has a spectrum includingat least one recognizable portion at a known wavelength in saidpredetermined wavelength band; filtering said optical transmissionsignal from said optical signal at said filter so that said opticaltransmission signal passes through said filter, wherein said filter is afilter tunable on several wavelengths in a search band including saidspectrum, by command means operable under several operating conditions,and said filtering comprises:scanning said predetermined wavelength bandby varying said operating conditions; identifying said recognizablespectrum portion; determining, based on the operating conditionscorresponding to said recognizable spectrum portion, a search range forsaid transmission signal; scanning said search range by varying saidoperating conditions; recognizing said optical transmission signal insaid search range and identifying the relevant operating conditions;maintaining the operating conditions at said optical transmissionsignal; and receiving in said receiving unit the optical transmissionsignal passing through said filter.
 2. An optical telecommunicationmethod according to claim 1, characterized in that said steps ofscanning said predetermined wavelength band and identifying saidrecognizabile spectrum portion comprise:actuating said command means atat least two operating conditions corresponding to through-wavelengthsof said filter included within said search band; detecting the opticalthrough-power values at each of said operating conditions; andidentifying between said optical power values a value corresponding tosaid recognizable portion of said spectrum and the operating conditionsthereof.
 3. An optical telecommunication method according to claim 1,characterized in that said step of determining a search range comprisesthe step of determining, starting from the operating conditions of saidrecognizable portion, new operating conditions corresponding to a rangeof said spectrum in which said optical transmission signal can beindividually present and applying said new operating conditions toactuation of said command means.
 4. An optical telecommunication methodaccording to claim 1 further comprising amplifying said signal at leastonce by at least one active-fiber optical amplifier having aspontaneous-emission spectrum in said band including at least one peakof known wavelength constituting said recognizable spectrum portion. 5.An optical telecommunication method according to claim 4, characterizedin that said active-fibre amplifier comprises an erbium-doped fibre. 6.An optical telecommunication system comprising:anoptical-signal-transmitting station comprising means for generatingtransmission signals at at least two wavelengths included in apredetermined bandwidth and means for conveying said signals to a singleoptical fiber line; a receiving station for receiving said opticalsignals; and an optical fiber line connecting said transmitting andreceiving stations, characterized in that said optical-signal-receivingstation comprises means for separating said transmission signals fromsaid single optical fiber line comprising: a signal splitter designed tosplit the incoming optical signal onto several optical outlets; at leastone tunable optical filter connected in series to at least one of saidoptical outlets, adapted to produce an outgoing optical output signal ina wavelength band of predetermined width based on optical wavelengthsignals present in a recognizable spectrum portion and comprisingrespective commandable actuator means; means for receiving at least oneportion of said outgoing optical output signal from said filter; andmeans for commanding said actuator means of said filter, in connectionwith said receiving means.
 7. An optical telecommunication systemaccording to claim 6 further comprising at least one active-fiberoptical amplifier interposed along said optical fiber line.
 8. Anoptical telecommunication system according to claim 7, characterized inthat said amplifier is an erbium-doped active-fibre amplifier.
 9. Anoptical telecommunication system according to claim 7, characterized inthat said tunable filter is a filter of the Fabry-Perot type.
 10. Anoptical telecommunication system according to claim 7, characterized inthat said receiving means designed to receive at least one portion ofsaid outgoing optical signal from said filter comprises a fused-fibresplitter, connected in series at the filter output, having an outletconnected with an optical check receiver.
 11. An opticaltelecommunication system according to claim 10, characterized in thatsaid optical check receiver comprises a photodiode for the electricdetection of the at least one portion of said outgoing optical signal.12. A device for a multi-wavelength optical reception comprising:asignal splitter adapted to split an incoming optical signal onto severaloptical outlets; at least one tunable optical filter connected in seriesto at least one of said optical outlets, adapted to produce an opticaloutput signal in a wavelength band of predetermined width based onoptical wavelength signals present in a recognizable spectrum portionand comprising respective commandable actuator means; means forreceiving at least one portion of said optical output signal from saidfilter; and means for commanding said actuator means for said filter, inconnection with said receiving means.
 13. A device for amulti-wavelength optical reception according to claim 12, characterizedin that said tunable filter is a filter of the Fabry-Perot type.
 14. Adevice for a multi-wavelength optical reception according to claim 12,characterized in that said means for receiving at least one portion ofsaid optical output signal from said filter comprises a fused-fibresplitter connected in series at its exit from the filter, having anoutlet connected to an optical check receiver.
 15. A device for amulti-wavelength optical reception according to claim 14, characterizedin that said optical check receiver comprises a photodiode for theelectronic detection of the at least one portion of said optical signal.16. An optical telecommunication method comprising the stepsof:generating at least one optical transmission signal, at apredetermined wavelength included in a predetermined wavelength band;transmitting said optical transmission signal through an optical fiberto a receiving station, comprising at least a receiving unit; feeding anoptical signal comprising said optical transmission signal to arespective one of the receiving units in said receiving station, througha passband filter, wherein said optical signal has a spectrum includingat least one recognizable portion at a known wavelength in saidpredetermined wavelength band; filtering said optical transmissionsignal from said optical signal at said filter so that said opticaltransmission signal passes through said filter, wherein said filter is atunable optical filter (Fabry-Perot) tunable on several wavelengths in asearch band including said spectrum, by command means operable underseveral operating conditions, wherein said command means is embodied bypiezoelectric actuators and wherein said filtering comprises:scanningsaid predetermined wavelength band by varying said operating conditions;identifying said recognizable spectrum portion; determining, based onthe operating conditions corresponding to said recognizable spectrumportion, a search range for said transmission signal; scanning saidsearch range by varying said operating conditions; recognizing saidoptical transmission signal in said search range and identifying therelevant operating conditions; maintaining the operating conditions atsaid optical transmission signal; and receiving in said receiving unitthe optical transmission signal passing through said filter.
 17. Anoptical telecommunication method according to claim 16, characterized inthat said filtering step comprises:applying to said actuators two ormore piloting voltages included between the extreme values ofpredetermined voltages; detecting the optical power values of the signalpassing through the filter at said voltages; recognizing saidrecognizable spectrum portion and the piloting voltage correspondingthereto; modifying said predetermined extreme voltage values dependingon the value of said piloting voltage corresponding to said recognizableportion; repeating the cycle a predetermined number of times;determining a search voltage range for a signal; recognizing said signalin said range; and maintaining said filter at said signal.
 18. Anoptical telecommunication method according to claim 17, characterized inthat said recognizing step comprises:applying to said actuators pilotingvoltages included in said search voltage range and detecting the opticalthrough-powers corresponding thereto; and recognizing as a signal eachmaximum value of the optical through-power.
 19. An opticaltelecommunication method according to claim 17, characterized in thatsaid maintaining step comprises applying to said actuators the pilotingvoltage corresponding to said recognized maximum of optical power andperiodically varying said voltage according to predetermined increments,by adopting the piloting voltage value corresponding to the detectedmaximum of optical through-power.
 20. An optical telecommunicationmethod according to claim 17, characterized in that said filtering stepcomprises varying the piloting voltage between said extreme values bymeans of a predetermined temporal law.
 21. An optical telecommunicationmethod according to claim 20, characterized in that said pilotingvoltage is varied according to increments fixed in time.
 22. An opticaltelecommunication method according to claim 21, characterized in thatsaid piloting voltage is varied according to a mean temporal gradientpredetermined in each step.
 23. An optical telecommunication methodaccording to claim 16, characterized in that said recognizable spectrumportion includes a spontaneous-emission peak of erbium, at a wavelengthincluded between 1530 and 1540 nm.
 24. An optical telecommunicationmethod comprising the steps of:generating at least one opticaltransmission signal, at a predetermined wavelength included in apredetermined wavelength band; transmitting said optical transmissionsignal through an optical fiber to a receiving station, comprising atleast a receiving unit; feeding an optical signal comprising saidoptical transmission signal to a respective one of the receiving unitsin said receiving station, through a passband filter, wherein saidoptical signal has a spectrum including at least one recognizableportion at a known wavelength in said predetermined wavelength band;filtering said optical transmission signal from said optical signal atsaid filter so that said optical transmission signal passes through saidfilter, wherein said filter is a tunable optical filter tunable onseveral wavelengths in a search band including said spectrum, by commandmeans operable under several operating conditions, and said filteringcomprises:scanning said predetermined wavelength band by varying saidoperating conditions; identifying said recognizable spectrum portion;determining, based on the operating conditions corresponding to saidrecognizable spectrum portion, a search range for said transmissionsignal; scanning said search range by varying said operating conditions;recognizing said optical transmission signal in said search range andidentifying the relevant operating conditions, wherein said recognizingcomprises:detecting the optical power passing through the filter in agroup of at least three consecutive operating conditions; separating anoptical through-power value detected at an intermediate operatingcondition between said consecutive operating conditions, from theoptical through-power values detected at at least two external operatingconditions, between which said intermediate condition is included;calculating an optical interpolation power value at said intermediateoperating condition; comparing said detected optical through-power valuewith said optical interpolation power value; recognizing as theoperating conditions corresponding to the optical transmission signal,the intermediate operating conditions in which said detected opticalthrough-power value and said optical interpolation power value are in apredetermined relation with respect to each other; maintaining theoperating conditions at said optical transmission signal; and receivingin said receiving unit the optical transmission signal passing throughsaid filter.
 25. An optical telecommunication method according to claim24, characterized in that said predetermined relation comprises a higherratio between said optical through-power value and said opticalinterpolation power value than a predetermined threshold value.
 26. Anoptical telecommunication method according to claim 24, characterized inthat said predetermined relation comprises a ratio between the integralof an interpolation curve of said optical through-power values detectedat said consecutive operating conditions and the integral of aninterpolation curve of said detected optical through-power values,except for the value or values corresponding to said intermediateoperating condition or conditions in said group, said ratio being higherthan a predetermined threshold value.
 27. An optical telecommunicationmethod comprising the steps of:generating at least one opticaltransmission signal, at a predetermined wavelength included in apredetermined wavelength band; transmitting said optical transmissionsignal through an optical fiber to a receiving station, comprising atleast a receiving unit; feeding an optical signal comprising saidoptical transmission signal to a respective one of the receiving unitsin said receiving station, through a passband filter, wherein saidoptical signal has a spectrum including at least one recognizableportion at a known wavelength in said predetermined wavelength band;filtering said optical transmission signal from said optical signal atsaid filter so that said optical transmission signal passes through saidfilter, wherein said filter is a filter tunable on several wavelengthsin a search band including said spectrum, by command means operableunder several operating conditions, wherein said filteringcomprises:scanning said predetermined wavelength band by varying saidoperating conditions; identifying said recognizable spectrum portion;determining, based on the operating conditions corresponding to saidrecognizable spectrum portion, a search range for said transmissionsignal, wherein the step of determining the search rangecomprises:determining, starting from the operating conditions of saidrecognizable portion, new operating conditions corresponding to a rangeof said spectrum in which said optical transmission signal can beindividually present; applying said new operating conditions to saidactuators; detecting a spontaneous-emission spectrum; identifying theoperating conditions corresponding to the extremes of saidspontaneous-emission spectrum; and calculating the operating conditionscorresponding to one portion of said spectrum in which said transmissionsignal can be individually localized; scanning said search range byvarying said operating conditions; recognizing said optical transmissionsignal in said search range and identifying the relevant operatingconditions; maintaining the operating conditions at said opticaltransmission signal; and receiving in said receiving unit the opticaltransmission signal passing through said filter.
 28. An opticaltelecommunication system comprising:an optical-signal-transmittingstation comprising means for generating transmission signals at at leasttwo wavelengths included in a predetermined bandwidth and means forconveying said signals to a single optical fibre line; a receivingstation for receiving said optical signals; an optical fibre lineconnecting said transmitting and receiving stations; wherein saidoptical-signal-receiving station comprises means for separating saidtransmission signals from said single optical fibre line comprising:asignal splitter designed to split the incoming optical signal ontoseveral optical outlets; at least one tunable optical filter connectedin series to at least one of said optical outlets, adapted to produce anoutgoing optical output signal in a wavelength band of predeterminedwidth and comprising respective commandable actuator means; means forreceiving at least one portion of said outgoing optical output signalfrom said filter comprising a fused-fibre splitter connected in seriesat the filter output and having an outlet connected with an opticalcheck receiver, wherein said fused-fibre splitter draws less than 5% ofoptical power to be sent to said optical check receiver; and means forcommanding said actuator means of said filter, in connection with saidreceiving means.
 29. An optical telecommunication system comprising:anoptical-signal-transmitting station comprising means for generatingtransmission signals at at least two wavelengths included in apredetermined bandwidth and means for conveying said signals to a singleoptical fibre line; a receiving station for receiving said opticalsignals; an optical fibre line connecting said transmitting andreceiving stations; wherein said optical-signal-receiving stationcomprises means for separating said transmission signals from saidsingle optical fibre line comprising: a signal splitter designed tosplit the incoming optical signal on several optical outlets; at leastone tunable optical filter of the Fabry-Perot type connected in seriesto at least one of said optical outlets, adapted to produce an outgoingoptical output signal in a wavelength band of predetermined width,having a free spectral range, FSR, greater than or equal to thespontaneous-emission band of said erbium-doped active fiber andcomprising respective commandable actuator means; means for receiving atleast one portion of said optical output signal from said filter; andmeans for commanding said actuator means of said filter, in connectionwith said receiving means.
 30. A device for a multi-wavelength opticalreception comprising:a signal splitter adapted to split an incomingoptical signal onto several optical outlets; at least one tunableoptical filter connected in series to at least one of said opticaloutlets, adapted to produce an optical output signal in a wavelengthband of predetermined width, comprising respective commandable actuatormeans; means for receiving at least one portion of said optical outputsignal from said filter; and means for commanding said actuator meansfor said filter, in connection with said receiving means, comprising amicroprocessor unit adapted to generate a command action on theactuators in response to the filter output signal.
 31. A device for amulti-wavelength optical reception comprising:a signal splitter adaptedto split an incoming optical signal onto several optical outlets; atleast one tunable optical filter connected in series to at least one ofsaid optical outlets, adapted to produce an optical output signal in awavelength band of predetermined width, comprising respectivecommandable actuator means, wherein said actuator means for said filterare piezoelectric actuators; means for receiving at least one portion ofsaid optical output signal from said filter; and means for commandingsaid actuator means for said filter, in connection with said receivingmeans.
 32. A device for a multi-wavelength optical reception,characterized in that it comprises:a signal splitter adapted to split anincoming optical signal onto several optical outlets; at least onetunable optical filter connected in series to at least one of saidoptical outlets, adapted to produce an optical output signal in awavelength band of predetermined width, comprising respectivecommandable actuator means; means for receiving at least one portion ofsaid optical output signal from said filter comprising a fused-fibresplitter connected in series at its exit from the filter and having anoutlet connected to an optical check receiver, wherein said fused-fibresplitter draws less than 5% of optical power to be sent to said opticalcheck receiver; and means for commanding said actuator means for saidfilter, in connection with said receiving means.
 33. A device for amulti-wavelength optical reception comprising:a signal splitter adaptedto split an incoming optical signal on several optical outlets; at leastone tunable optical filter of the Fabry-Perot type connected in seriesto at least one of said optical outlets, adapted to produce an opticaloutput signal in a wavelength band of predetermined width, having a freespectral range FSR greater than or equal to said predetermined bandwidth and comprising respective commandable actuator means; means forreceiving at least one portion of said optical output signal from saidfilter; and means for commanding said actuator means for said filter, inconnection with said receiving means.